1. Field of the Invention
This invention relates to a field effect transistor serving as a main component of an integrated circuit element and, more particularly, a field effect transistor having a strain distribution channel.
2. Description of the Related Art
Recently, technical and economic barriers to reduction of a gate length in a field effect transistor (MOSFET) have been radically higher. To relax this condition, a technique of using a channel material with high mobility and, particularly, strained-Si and strained-SiGe, has been noticed.
Strained SiGe is a SiGe crystalline thin film which is in a state of lattice matching on a Si substrate, and has compressive strain in an in-plane direction of the substrate. Due to influences of the compressive strain, a band structure is varied and, particularly, hole mobility increases as compared with Si. In this case, as Ge composition is higher, the strain increases and the hole mobility becomes higher. On the other hand, strained Si is formed on lattice-relaxed SiGe having a greater lattice constant, and has a tensile strain in the in-plane direction of the substrate. Due to influences of the tensile strain, the band structure is varied and, both the electron mobility and the hole mobility increase as compared with Si. In this case, as the Ge composition of the base SiGe is greater, an amount of the stain in the strained-Si becomes greater and the mobilities become higher.
If a CMOS is composed of MOSFETs having the strain-SiGe or strained-Si channels, more high-speed operations can be expected than a Si-CMOS of the same size.
The present inventors have proposed a MOSFET (a strained-SOI-MOSFET) obtained by combining strained SiGe or strained Si with the SOI (Si-On-Insulator) structure and demonstrated its operations (T. Tezuka, N. Sugiyama, T. Mizuno and S. Takagi, IEDM Tech. Dig. 946 (2001), T. Mizuno, S. Takagi, N. Sugiyama, J. Koga, T. Tezuka, K. Usuda, T. Hatakeyama, A. Kurobe, and A. Toriumi, IEDM Technical Digests p. 934 (1999)).